For example, in producing a metal plate, such as a cold-rolled steel sheet, a process of rolling an intermediate product (e.g., a hot-rolled steel sheet) is performed. In such a rolling process, stretching is performed using a rolling mill roll so that the final product has a desired thickness. In the case where a foreign substance is attached to the rolling mill roll, the foreign substance may cause an unevenness flaw on the surface of the metal plate. In the case where the rolling mill roll is under micro vibration, the vibration may cause fine lateral stripes (chatter marks) to be formed on the surface of the metal plate. Furthermore, dirt on a production line or the like may cause harmless dirt to be attached to the surface of the metal plate.
The amount of unevenness of this unevenness flaw is from approximately 1 μm to approximately 10 times larger than that. On the other hand, the surface of a steel strip in a production process before coating has a roughness of approximately 0.5 μm to 1 μm, which is comparable to visible light; thus, a diffuse reflection component is large in visual inspection by visible light, and it is difficult to find unevenness defects. To detect such unevenness defects, conventionally, visual inspection has been performed after honing had been performed on a metal plate by an inspector. When honing is performed on a metal plate, a convexity is polished further than a concavity to become closer to a specular surface, whereas the concavity remains as the original rough surface; thus, a part with unevenness becomes clear to be visually checkable. However, there has been a problem in that time and effort are taken for visually checking presence or absence of an unevenness flaw on a produced metal plate.
To solve such problems, studies have been carried out for a method of detecting micro defects present on the surface of a steel sheet without visual check, by using light belonging to the infrared wavelength band (infrared light). For example, Patent Literature 1 below discloses a method of detecting micro unevenness flaws present on the surface of an inspection target by applying infrared light to the inspection target, projecting reflected light from the inspection target on a screen, and observing light and dark with a camera. In addition, Patent Literature 2 below discloses a method of applying infrared laser light made into divergent light to an inspection object, and imaging, with a camera, reflected light from the inspection object condensed by a concave mirror.